Determining whether a query is appropriate to hit the read replica is something I've never seen done automatically and well. But if you design your system from the beginning that when you make queries the developer chooses whether to hit the reader or writer by way of essentially do_query / do_query_reader then it's a million times easier scale reads and it doesn't have to come with any additional operational complexity when you're small you can just have both handles point to the writer but have a read-only user or set TRANSACTION READ ONLY on the session.